The generally used internal combustion engine is designed to set a valve overlap period in which both the intake valve and the exhaust valve are simultaneously opened around the intake top dead center in order to improve combustion performance. Concurrent opening of both the intake valve and the exhaust valve causes reverse flow of the combustion gas into the intake port. The reverse flow increases residual gas concentration in the compression stroke, resulting in deterioration both in the combustion performance and the exhaust gas emission performance.
In order to compensate for the above-described deterioration, a variable valve train for varying the valve lift amounts or the valve timings is employed. At low rotational speed, the variable valve train operates to reduce the valve overlap period for preventing reverse flow of the combustion gas into the intake port so as to improve both the combustion performance and the emission performance. At high rotational speed, the variable valve train operates to prolong the valve overlap period for improving the output performance.
On the other hand, in order to prevent noise around the valve train system, the valve lift curve is required to be a sufficiently long buffer curve for reducing noise generated upon seating of the intake valve and the exhaust valve. In addition to the above, excessive reduction in the valve overlap period in the aforementioned state results in failing to ensure sufficient valve opening period and sufficient valve lift amount, resulting in disadvantage of reduced output of the engine.
In case of employing the variable valve train of cam nose switchable type, switching of the cams causes difference in the valve lift curve, between a cam selected to improve emission performance, and a cam selected to improve output performance. The use of the variable valve train of the above-described type therefore causes an output discontinuity.
In the case of employing the variable valve train of cam phase changing type, the effect of improving both the output performance and the emission performance is limited owing to its structurally limited phase changing angle. Further, the valve timings during the period other than the valve overlap period are made inappropriate, resulting in deteriorated thermal efficiency.
The use of the variable valve train causes a cost increase, and further causes such problems as complication of the entire system of the valve train, and lowered rotational speed limit.